Collaborative writing combines social processes of writing with cognitive knowledge construction processes, and thus may lead to deeper learning than individual working. This study examined students’ knowledge construction during face-to-face collaborative writing. University students (n = 21) prepared themselves for the collaborative task by reading about developmental theories in a course book and writing individual summaries of them. In small groups, the students discussed each others’ summaries and wrote a joint essay on one of the theories. The data comprise the students’ individual summaries (n = 21), the students’ discussions during the essay writing (8177 speech turns), and the students’ joint essays (n = 6). The utterances from the students’ discussions indicating knowledge construction (n = 3865) were broadly categorized under the headings of collaborative interaction (Asking questions, Answers to questions, Collaborative completion, Expressing disagreement or conflict, Quick agreement, Quick disagreement and Discussing edits) and content processing (Conceptualizing ideas; Clarifying ideas and Expressing an idea or a thought). When constructing knowledge during collaborative writing, the students mainly completed each others’ ideas and asked questions. They rarely expressed disagreement or conflict. A2 collaborative writing task that combines reading, summary writing and group discussion seems to assist students in their construction of knowledge.